In recent years, nonaqueous electrolyte secondary batteries are often used as main power sources of mobile apparatuses such as mobile telecommunications devices and portable electronic devices since they can provide high energy density at high voltage. There are also demands recently for nonaqueous electrolyte secondary batteries of light-weight and small size, yet capable of delivering large discharge currents because of the needs for installing them in automobiles, and for use with direct current-driven heavy tools.
Despite of these demands, nonaqueous electrolyte secondary batteries usually result in temperature rises due to increase in heat generated by the Joule effect when large currents are discharged since the batteries have internal direct current resistances. Generally, nonaqueous solvent in an electrolytic solution used for the nonaqueous electrolyte secondary batteries contains a component that boils or resolves when the temperature exceeds about 90° C. For this reason, charge and discharge capacity decreases drastically when repeating such a cycle that causes the battery temperature to exceed 90° C. during charging and discharging. Numerous studies are being made in efforts to solve such a problem.
An internal resistance of any battery is divided into a reactive resistance related to reaction of the battery, a resistance attributable to the electrolytic solution and separator, and a resistance of current collectors. In order to lower the resistance of current collectors among these resistances, there is a work disclosed in Japanese Patent Unexamined Publication, No. H11-233148, for example, which decreases the direct current resistance of battery by improving a structure of connections of a positive electrode and a negative electrode to exterior parts. This work is aimed at reducing the Joule heat generated inside the battery. An electric power (i.e., output power) is the product of a current and a voltage. Therefore, in the case of an apparatus requiring a high power by way of constant power discharge, such as a power tool, there occurs a rise in the discharge rate (i.e., discharge current) when the voltage decreases quickly near the end of electric discharge. Since the decrease in voltage is attributed to a material of the positive electrode, the above technique of decreasing the direct current resistance does not provide a direct effect in this case.
There is also an idea of installing a temperature sensor on a surface of the battery, and using a control to stop operation of an apparatus when a surface temperature of the battery reaches a predetermined value or higher, as disclosed in Japanese Patent Unexamined Publication, No. 2004-179085, for example. Besides the electrolytic solution, however, nonaqueous electrolyte secondary batteries contain other materials that generate heat under high temperatures, such as an active material for positive electrode in the end of electric discharge. In other words, the active material for the positive electrode generates a large amount of heat by reaction if it is discharged to a low voltage potential. There is thus a possibility that the battery becomes overheated if the predetermined control temperature is set too high in the above control. On the other hand, the discharge capacity decreases drastically if the predetermined control temperature is set too low.
In addition, there is another idea of deterring the drastic decrease in voltage near the end of electrical discharge by using two kinds of active materials having different ranges of average discharge voltage for the positive electrode as disclosed, for example, in Japanese Patent Unexamined Publication, No. H09-180718. In an example that uses such active materials for the positive electrode, there gives rise to a problem described hereafter when a large current is discharged as in the case of power tools. That is, the battery comes to a discharge-end voltage due to a rise in electrical potential of the negative electrode in reality at the end of electrical discharge, even though it is intended to bring the battery voltage to the discharge-end voltage by decreasing an electrical potential of the positive electrode. This impedes the effect of voltage control by means of the discharge voltage in the positive electrode, and thereby it makes the battery in the state of overheating near the end of electric discharge. As a result, it makes a reduction of designed capacity since it becomes necessary to increase an irreversible capacity of the positive electrode larger than that of the negative electrode in order to avoid this problem.